1. Field of the Invention
The present invention relates to an information transmission system and an information transmission method for transmitting information through multiple terminals, and in particular to a technique for reducing the load imposed on information transmission traffic.
2. Related Art
Recently, a large variety of electronic apparatuses and electric facilities have been developed and are in practical use, and these apparatuses appear intelligent because of the development of microcomputer techniques. Further, as network techniques have been advanced, attempts have been made to provide a convenient employment form whereby these intelligent apparatuses can be interconnected, and exchange information and communicate with each other.
The “Bluetooth standards” and IEEE 802.11B have been advocated as communication standards for interconnecting these apparatuses. The Bluetooth standards are radio communication standards that use a radio frequency of 2.4 GHz, and one of the techniques used for the transmission of messages to fixed terminals or mobile terminals. According to the Bluetooth standards, a source terminal can broadcast messages to other terminals that are present within communication range, so that the messages can be received by many, unspecified terminals. The communication range of a terminal (i.e., the range within which a radio wave can received) is about 10 m at Class C.
Since no broadcast transmission destinations are specified, unnecessary transmissions may be broadcast and relayed. Such unnecessary transmissions result in an increase in the overall amount of communications within the system, and the deterioration of the efficiency of message transmission. Thus, the following methods can be employed to reduce the transfer of unnecessary messages and to improve the efficiency of message transmission.
A first method is one whereby limitations are placed on the relaying of messages. Provided in the header of a message is a portion for recording the relay count, and when the relay count exceeds a predetermined maximum count, the terminal that last receives the message does not retransmit it.
A second method is one whereby limitations are placed on message relay distances. Before a message is broadcast, included in the header of the message is positional information for the transmission source terminal, and a terminal that receives the message uses the positional information for the source terminal and for the local terminal to calculate a message transmission distance. Or, a portion for recording an accumulated transfer distance is provided in the header, and when the recorded distance exceeds a predetermined maximum relay distance, the terminal that last receives the message does not retransmit it.
A third method is one whereby limitations are placed on message relay periods. Before a message is broadcast, included in the header of the message is the transmission time at the source terminal, and a terminal that receives the message uses the transmission time and the current time to calculate a message relay period. When the message relay period exceeds a predetermined maximum relay period, the terminal that last receives the message does not retransmit it.
When, as in these methods, the transfer count, the transfer distance or the transfer period is limited, a phenomenon can be avoided whereby a message that has been adequately transmitted is forwarded endlessly.
A fourth method is one whereby space is provided in a message for terminal identifiers and for accumulating route information from each terminal that forwards the message. Since route information is stored at each terminal, the overlapping transfer of information can be prevented by referring to the route information.
A fifth method is one whereby a unique identifier is provided for each message and for the storage of a history of the information that is transmitted to each terminal. Since each terminal stores history information, when a message is received that has the same identifier as a previous one, a terminal, by referring to the history information, can ascertain that the message is not to be forwarded. As a result, the overlapping transfer of information can be avoided.
A sixth method is one whereby a multicast algorithm (routing algorithm) is introduced to each terminal. When the multicast algorithm is defined in advance before being stored in each terminal, the terminal can employ the algorithm to determine whether a message should be forwarded. Thus, the overlapping transfer of information can be avoided. This method is described in detail in “Distance Vector Multicast Routing Protocol”, D. Waitzman, et. al, IETFRFC 1075, November 1988, or in “Core Based Tree (CBT)”, T. Ballardie, et. al., ACM SIGCOMM93, pp. 85-95, 1993.
A seventh method is one whereby whether information should be forwarded is determined by referring to density information for a terminal, a progress vector from a source terminal to a relay terminal and a progress vector from the source terminal to an addressed terminal. When the density information for a terminal, the progress vector from a source terminal to a relay terminal and the progress vector from the source terminal to an addressed terminal are referred to, the information can be radially forwarded from the source terminal, and a transfer that is the reverse of the intended information progression can be prevented. Since a transfer in the direction that is the reverse of the information progression direction is generally an unnecessary transfer, such an unnecessary transfer can be prevented by inhibiting reverse transmissions.
However, the following problems are encountered when the above methods are employed to reduce the volume of unnecessary communications.
According to the first, the second and the third methods, it is difficult to determine an appropriate maximum relay count, an appropriate maximum relay distance and an appropriate maximum relay period, and when the maximum count, distance or period is too small, a message may not be transmitted. Assume that, since a broadcast and a relay system itself may not be established if the message transmission is disabled, a satisfactorily large maximum is set in order to acquire a message transmission function. In this case, the transfer efficiency can not be improved, and the objective can not be achieved.
According to the fourth method, a unique identifier must be provided for a terminal. This method can be applied for a closed system; however, for an open system, a globally unique identifier must be provided. For example, an identifier such as an IP (Internet Protocol) identifier is required, and a great load is imposed on maintenance and management.
According to the fifth method, history data for an identifier unique to a message should be held by each terminal, and as many history data as possible must be stored in order to improve transmission efficiency using this method. However, it is preferable that a terminal dedicated for broadcasting and relaying have as low a cost as possible and carry only a small load when performing information processing and information recording (according to the Bluetooth standards, it is assumed that these terminal functions will be carried out by a one-chip semiconductor device). Therefore, it is not preferable that a terminal having such a small load hold many history data. Further, as well as the determination of maximum values for the first to the third methods, it is difficult to determine an appropriate amount of history data to be stored. In addition, it is predicted that when there has been an enormous increase in the amount of communication, accordingly, the history data that will have to be stored will suffer a like increase. Therefore, when taking into account the processing form that will be required to cope with a huge increase in the amount of communication, it is apparent that the employment of this method is not advisable.
According to the sixth method, the establishment of a network topology is premised. So long as the network topology is established, unnecessary transfers can be effectively restricted by using this method. However, if the network topology is not established or is changed dynamically, this method can not be employed. For mobile communications, for example, dynamic network topology changes occur regularly. When the power supplied to a terminal is cut off, or when a terminal is powered on and connected to a network, the network topology at that time is not established.
Since according to the seventh method the premise is that there will be a fixed terminal, this method may not be satisfactory for a mobile terminal. Further, since a further premise is that density information will be acquired, the costs involved in calculating the density are high. If this method is employed on the assumption that a fixed terminal will be used, although the calculation of the density must only be performed once when the network is changed, if the method is applied for a mobile terminal, it is assumed that the terminal density is constantly changing, and the terminal density must be calculated in real time. In this case, the processing load imposed on each terminal to obtain the density information can not be ignored, and to measure the terminal density the communication load must be increased.